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Re: Software PLL (SPLL)

Started by Noway2 February 7, 2006
Thank you both.  I think I understand things a bit better than before.

I think the idea of using a gross zero crossing aproximator to drive
the PLL into the neighborhood of the incoming frequency and then
letting the loop filter / NCO take over sounds like a very workable
idea.

Tim, as I indicated, I have been planning this as a future sub project
for a while now.  I have played around with the PFD concept multiplying
a reference and "line" (sine and cosine waves at various magnitudes,
frequencies and phase offsets)  and filtering the result in mathcad to
get an intuitive feel for the operation and effects.  From these
experiments, I believe I understand what you are saying about the phase
error I and Q rotating (like a vector spinning in time) and how
depending on the phase relationship of the signals one will be high,
the other will be low.

The big part that I haven't addressed in time is the part about driving
the NCO.  I realize that the output of the loop filter does this, but I
suspect part of the trick will be determining the gain or attenuation
factor to the NCO, but that can wait till another day.